Led switch circuitry for varying input voltage source

ABSTRACT

An LED array switching apparatus, comprises: a plurality of LED arrays arranged in a serial path; a voltage supply coupled to the plurality of LED arrays; a plurality of current sources selectively coupled to the LED arrays, each of the current sources being switchable between a current regulating state and an open state; and a controller that outputs at least one control signal. The controller, the at least one switch and current sources cooperate together such that: when the voltage of the voltage source is below the at least one reference voltage, and/or when a predetermined level of current passes through the one or more current sources, at least one switch is closed and one or more associated current sources are controlled so as to break the serial path into one or more parallel paths each including less than all of the LED arrays.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation-in-Part of U.S. application Ser. No.12/955,030, filed Nov. 29, 2010, which claims benefit of U.S.Provisional Patent Application No. 61/373,058, filed Aug. 12, 2010, theentirety of each of which are incorporated by reference herein.

BACKGROUND OF THE INVENTION

The present invention relates to switching circuitry used in driving LEDlight sources. In particular, circuitry in which LEDs are driven by aregulated current source.

Conventionally, LEDs may be driven by a current source that regulatesthe current flowing through the LEDs and hence maintains the lightoutput of the LEDs. FIG. 1 shows a typical circuit for driving an LEDcircuit in which V is an input voltage source, D is representative of astring of LEDs and G is a current source. In such a circuit, in orderfor current to flow through D, the source input voltage of V must behigher than the forward voltage of the LEDs D.

However, if voltage of input voltage source V is much higher than theforward voltage of D, a large voltage drop is present in current sourceG. Such an occurrence may cause a significant power loss in currentsource G, particularly if current source G is a linear current source.

BRIEF SUMMARY OF THE INVENTION

In accordance with a first aspect of the present invention, an LED arrayswitching apparatus comprises: a plurality of LED arrays arranged in aserial path, each LED array having a forward voltage; a voltage supplycoupled to the plurality of LED arrays; a plurality of current sourcesselectively coupled to the LED arrays, each of the current sources beingswitchable between a current regulating state and an open state; and acontroller that outputs at least one control signal generated based onat least one of: (a) at least one comparison between the voltage of thevoltage supply and at least one reference voltage, and (b) currentsthrough one or more of the current sources, the control signalscontrolling the turning on and off of at least one switch and a currentsource associated with the at least one switch. The controller, the atleast one switch and current sources cooperate together such that: whenthe voltage of the voltage source is below the at least one referencevoltage, and/or when a predetermined level of current passes through theone or more current sources, at least one switch is closed and one ormore associated current sources are controlled so as to break the serialpath into one or more parallel paths each including less than all of theLED arrays.

In another aspect, for at least a portion of time during which thevoltage of the voltage supply is below at least one reference voltage,the one or more parallel paths comprise a plurality of parallel pathseach including at least one of the LED arrays, the plurality of parallelpaths supplying current to all of the LED arrays.

In another aspect, the LED array switching apparatus further comprises:at least one diode arranged in the serial path of the LED arraysintermediate between a first group of LED arrays and a second group ofLED arrays; and a switchable parallel current path that connects thevoltage supply to a point in the serial path between the diode and thesecond group of LED arrays, the at least one diode preventing currentfrom the parallel current path from flowing in the direction of thefirst group of LED arrays.

In another aspect, the number of LED arrays in the first group of LEDarrays is equal to the number of LED arrays in the second group of LEDarrays.

In another aspect, a plurality of parallel paths supplies current to allof the LED arrays when the voltage of the voltage source is higher thanthe forward voltage of both of the first or second group of LED arrays,but is less than the at least one reference voltage.

In another aspect, the voltage source is a rectified AC voltage, and theswitching apparatus further comprises: valley-fill circuitry configuredto prevent occurrence of any off period of light output at a zerocrossing portion of the AC voltage.

In another aspect, the valley-fill circuitry includes at least oneenergy storage capacitor that discharges when the rectified AC voltagedrops below half its peak value to prevent any off period of the lightoutput.

In another aspect, at least one of the plurality of LED arrays comprisesa plurality of LEDs.

In another aspect, the plurality of LEDs forming the at least one of theplurality of LED arrays are arranged in parallel.

In another aspect, the controller comprises one or more voltagecomparators.

In another aspect, the controller comprises a microcontroller.

In another aspect, the microcontroller is configured to detect a faultin an LED array and modify a switching sequence to exclude the faultedLED array.

In another aspect, the at least one reference voltage is a plurality ofreference voltages, and the at least one switch is a plurality ofswitches, and each of the plurality of reference voltages correspondswith a respective one of the switches.

In accordance with a second aspect of the present invention, an LEDarray switching apparatus comprises: a plurality of LED arrays arrangedin a serial path, each LED array having a forward voltage; a voltagesupply coupled to the plurality of LED arrays; a voltage comparator thatcompares the voltage of the voltage supply with a reference voltage andcontrols a switch to turn off when the voltage of the voltage supply isgreater than or equal to the reference voltage; and a plurality ofcurrent sources selectively coupled to the LED arrays each of thecurrent sources is switchable between a current regulating state and anopen state. The voltage comparator, the switch and current sourcescooperate together such that: (a) when the voltage of the voltage sourceis below the reference voltage, the switch is closed and the currentsources are controlled so as to break the serial path into one or moreparallel paths each including less than all of the LED arrays, and (b)when the voltage of the voltage supply is greater than or equal to thereference voltage, as the voltage of the voltage supply increases, LEDarrays are switched on and lit to form a higher forward voltage LEDstring, and as the voltage of the voltage supply decreases, LED arraysare switched off and removed from the LED string starting with the mostrecently lit array.

In another aspect, for at least a portion of time during which thevoltage of the voltage supply is below the reference voltage, the one ormore parallel paths comprise a plurality of parallel paths eachincluding at least one of the LED arrays, the plurality of parallelpaths supplying current to all of the LED arrays.

In another aspect, the LED array switching apparatus further comprises:a diode arranged in the series path of the LED arrays intermediatebetween a first group of LED arrays and a second group of LED arrays;and a switchable parallel current path that connects the voltage supplyto a point in the series path between the diode and the second group ofLED arrays, the diode preventing current from the parallel current pathfrom flowing in the direction of the first group of LED arrays.

In another aspect, the number of LED arrays in the first group of LEDarrays is equal to the number of LED arrays in the second group of LEDarrays.

In another aspect, a plurality of parallel paths supplies current to allof the LED arrays when the voltage of the voltage source is higher thanthe forward voltage of both of the first or second group of LED arrays,but is less than the reference voltage.

In another aspect, the voltage source is a rectified AC voltage, and theswitching apparatus further comprises: valley-fill circuitry configuredto prevent occurrence of any off period of light output at a zerocrossing portion of the AC voltage.

In another aspect, the valley-fill circuitry includes at least oneenergy storage capacitor that discharges when the rectified AC voltagedrops below half its peak value to prevent any off period of the lightoutput.

In another aspect, the sum of the forward voltages of LED arrays in thefirst group of LED arrays is approximately equal to sum of the forwardvoltages of the LED arrays in the second group of LED arrays.

In another aspect, at least one of the plurality of LED arrays comprisesa plurality of LEDs.

In another aspect, the plurality of LEDs forming the at least one of theplurality of LED arrays are arranged in parallel.

BRIEF DESCRIPTION OF THE DRAWINGS

The figures are for illustration purposes only and are not necessarilydrawn to scale. The invention itself, however, may best be understood byreference to the detailed description which follows when taken inconjunction with the accompanying drawings in which:

FIG. 1 is a circuit diagram of a conventional LED driving circuit thatutilizes a current source;

FIG. 2 is functional block diagram of a circuit for LED array switchingin accordance with an embodiment of the present invention;

FIGS. 3A-3F are diagrams illustrating current paths taken through thecircuit of FIG. 2 at different voltages levels of the source voltage, inaccordance with an embodiment of the present invention.

FIG. 4 is a functional block diagram of the circuit of FIG. 2 with anoptional set of current sources for averaging of the usage among theLEDs, in accordance with an aspect of the present invention.

FIG. 5 is a circuit diagram showing a practical implementation of thecircuit shown in FIG. 2.

FIG. 6 is a diagram of the voltage waveform across nodes A and B in FIG.5.

FIG. 7 is a diagram of the current through element M1 in FIG. 5.

FIG. 8 is a diagram of the current through element M2 in FIG. 5.

FIG. 9 is a diagram of the current through element M3 in FIG. 5.

FIG. 10 is a diagram of the current through element DX1 in FIG. 5.

FIG. 11 is a diagram of the current through element DX3 in FIG. 5.

FIG. 12 is a diagram of the current through element DX4 in FIG. 5.

FIG. 13 is a diagram of the light output waveform of the circuit in FIG.5.

FIG. 14 is a diagram showing the input waveform at the AC main source inFIG. 5.

FIG. 15 is a circuit of a bleeder circuit that can be used with thecircuit of FIG. 5.

FIG. 16 is functional block diagram of a circuit for LED array switchingin accordance with a second embodiment of the present invention.

FIG. 17 is functional block diagram showing how a microcontroller can beused with the circuit of FIG. 16.

FIG. 18 is functional block diagram of an example circuit for LED arrayswitching in accordance with the second embodiment of the presentinvention.

FIGS. 19A-19G are diagrams illustrating current paths taken through thecircuit of FIG. 18 at different voltages levels of the source voltage,in accordance with the second embodiment of the present invention.

FIG. 20 is a circuit diagram showing a practical implementation of thecircuit shown in FIG. 18.

FIG. 21 is a diagram of the rectified mains voltage in FIG. 20.

FIG. 22 is a diagram that shows the LED arrays that are conductingduring a half AC cycle.

FIG. 23 is a diagram of the current through element D1 in FIG. 20.

FIG. 24 is a diagram of the current through element D2 in FIG. 20.

FIG. 25 is a diagram of the current through element D3 in FIG. 20.

FIG. 26 is a diagram of the light output waveform of the circuit in FIG.20.

FIG. 27 is a diagram of the current of the AC mains source.

FIG. 28 is a diagram of an exemplary valley-fill passive power factorcorrection circuit.

FIG. 29 is a diagram of the mains voltage waveform with the valley-fillcircuit.

FIG. 30 is a diagram of the current through element D1 with thevalley-fill circuit.

FIG. 31 is a diagram of the current through element D2 with thevalley-fill circuit.

FIG. 32 is a diagram of the current through element D3 with thevalley-fill circuit.

FIG. 33 is a diagram of the light output waveform of the circuit withthe valley-fill circuit.

FIG. 34 is a current waveform of the AC mains source with thevalley-fill circuit.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 2-34 illustrate aspects of preferred embodiments of LED arrayswitching apparatus. For an LED lighting device to work using a varyinginput voltage source, such as a rectified AC source, the switchingapparatus in accordance with the first embodiment of the presentinvention divides the LED string into a series of multiple arrays. Whenthe input voltage is low, only the first LED array is lit up. As theinput voltage increases, subsequent LED arrays are switched in series toform a higher forward voltage string. Contrarily, if the input voltagedecreases, the sequence is reversed and arrays are removed from thestring starting with the last light-up array.

FIG. 2 shows the functional blocks of proposed circuitry. It is assumedthat the LED string is divided into n LED arrays or arrays D1 to Dn,where n>1. Each LED array may consist of one or more LEDs arranged inany know manner, i.e., in sequence or in parallel, or combinationsthereof G1 to Gn are current sources which can be disabled, that is,changed to an open circuit condition, by current sense signals fromsuccessive current sources.

The operation of the circuit of FIG. 2 is next described makingreference to FIGS. 3A-3F, for the case in which the voltage of V1 isramping up from zero. When the voltage of V1 is just above the forwardvoltage of LED array D1, current begins to flow through LED array D1 andcurrent source G1, as shown in FIG. 3A. Current source G1 regulates thecurrent through LED array D1 as voltage of V1 is further increased. LEDarray D2 begins to conduct when V1 reaches the sum of the forwardvoltages of LED array D1 and LED array D2, as shown in FIG. 3B. As thecurrent through LED array D2 is increasing to a threshold value, whichis preferably set lower than the regulating value of current source G2,current source G1 is disabled, becoming an open circuit. The currentthrough LED array D1 and LED array D2 is then regulated by currentsource G2, as shown in FIG. 3C.

FIG. 3D shows the current path in the circuit when V1 has been increasedto the point at which current source Gn−1 regulates the current throughLED arrays D1 to Dn−1. Further increasing V1 causes LED array Dn toconduct, as shown in FIG. 3E. FIG. 3F shows the current path when thecurrent through LED array Dn is increased to trigger current sources G1to Gn−1 to be in the open condition.

As would be understood by one of ordinary skill in the art, theswitching sequence shown in FIGS. 3A-3F would be reversed if the voltageof V1 is declining. In particular, the situation in which the voltage ofV1 is high enough to pass a regulated current through LED arrays D1 toDn and current source Gn is shown in FIG. 3F. As V1 is decreased, thecurrent through Gn starts to decrease and to a point below the thresholdvalue, current source Gn−1 is enabled and current begins to flow throughcurrent source Gn-1 as shown in FIG. 3E. When V1 decreases to a valuebelow the sum of forward voltage sum of LED arrays D1 to Dn, currentthrough LED array Dn is stopped, as shown in FIG. 3D.

As can be seen from the foregoing description, in the circuit of FIG. 2,LED array D1 conducts if any one of the current sources is conducting.On the other hand, LED array Dn only conducts if current source Gn isconducting. Thus, in operation, LED array D1 would be used more oftenthan LED array Dn. FIG. 4 is a block diagram of a circuit that averagesthe usage among LED arrays D1 to Dn. The circuit includes a set ofadditional current sources GT1-GTn and a current source set toggleswitcher TS1 added to the circuit of FIG. 2.

As can be seen in FIG. 4, the current source set toggle switcher TS1 hastwo complementary signal outputs Q and Q. Preferably, the toggleswitcher TS1 is configured such that these outputs are toggling atfrequency above 20 Hz, to avoid the perception of flicker. When Q of thetoggle switcher TS1 is active, the switch ST1 connected to this outputbecomes closed, current sources GT1 to GTn are disabled, and switch S1is opened. In this condition, the circuit of FIG. 4 is essentiallyidentical to the circuit shown in FIG. 2, and operates as describedabove upon occurrence of ramping up or down of input voltage V1.

When Q becomes active, and Q becomes non-active, switch S1 becomesclosed, current sources G1 to Gn are disabled, switch ST1 is opened, andcurrent sources GT1 to GTn are operational. In this situation, if V1 isramping up from zero voltage, unlike in the circuit of FIG. 1, Dn willbe the first conducting array followed by Dn−1, just the opposite ofwhat occurs in the circuit of FIG. 2. Thus, over time, the usage of theLEDs will average out.

FIG. 5 shows a practical detailed implementation of the proposed circuitshown in FIG. 2 with n=3. In the figure, the AC 220V main voltage sourceis a rectified signal. The voltage waveform across node A and B is shownin FIG. 6. The LED string, consists of four LEDs DX1-DX4, with forwardvoltage of 50V each, and is divided into 3 arrays. The first array has 2LEDs (DX1 and DX2) while the second and third arrays, each have a singleLED (DX3 and DX4, respectively).

As can be seen in the figure, transistor M1, resistors R1 and R11,transistor Q1 and diode D1 form a current source that drives LEDs DX1and DX2. Transistor Q11 turns off transistor M1 when the current throughtransistor M2 reaches threshold value.

FIG. 7 shows the current waveform of transistor M1. Waveformscorresponding to the current in transistors M2 and M3 are shown in FIGS.8 and 9, respectively. FIGS. 10, 11 and 12 show the current waveforms ofLEDs DX1, DX3 and DX4 respectively. The current of LED DX1 is thecurrent sum of transistors M1, M2 and M3, while the current of LED DX3is the current sum of transistors M2 and M3. FIG. 13 shows the lightoutput waveform of all the LED arrays.

FIG. 14 shows the input current waveform from AC main power source.Throughout most of the half line cycle, the current is continuous, whichmakes the circuit suitable to work with an optional triac dimmer, shownin FIG. 5. An optional bleeder circuit can be added to provide a currentpath for the triac dimmer's RC timing circuit when the triac is off.FIG. 15 shows a form of bleeder circuit which connects to node A and Bof FIG. 5. The bleeder circuit acts like a resistive load for the dimmerwhen the triac is not conducting. A bypass resistor 110 is switched onby transistor 2N60 to connect across the rectified input voltage whenthe rectified input voltage is low (which indicates the triac is off).With the bypass resistor completing the circuit, sufficient chargingcurrent can be supplied to the internal RC timing circuit of the triacdimmer to ensure proper operation. When the rectified input voltage ishigh (which indicates the triac is on), the bypass resistor isdisconnected by transistor 2N60 to minimize wasteful power dissipation.

In the first embodiment at low levels of input voltage, only the firstand second arrays D1 and D2 conduct. This condition results in a loweredlight output current waveform during low levels of input voltage, as canbe seen in FIG. 13 discussed above. A second embodiment of an LEDswitching apparatus is described with reference to FIGS. 16-34. Thesecond embodiment provides a time period at low input voltage in whichall of the LED arrays conduct current, in parallel branches, whichalleviates the abovementioned problem shown in FIG. 13. FIG. 16 showsthe functional blocks of a circuit for LED switching in accordance withthe second embodiment.

In the circuit shown in FIG. 16, V1 is a varying DC voltage source. D1to Dn are LED arrays, each of which can be more than one LED, formed inseries or parallel or combinations of serial and parallel. G1 to Gn arecurrent sources. S1 to Sn are switches. Db1 to Dbn are diodes. Eachsingle diode Dbi, where i can be 1 to n, functions to prevent currentthrough switch Si to current source Gi when switch Si is switched on.Control signal CSi is used to select either conducting state or opencircuit state of both switch Si and current source Gi.

When CS1 to CSn are not activated, switches S1 to Sn−1 are off andcurrent sources G1 to Gn−1 are in open circuit condition. All LED arraysD1 to Dn are series connected through diodes Db1 to Dbn and current iscontrolled by current source Gn. In this situation, if V1 is lower thanthe total forward voltage of D1 to Dn, the LED arrays will not be lit.However, in accordance with the disclosed embodiment, this low voltagecondition can be sensed, for example by a controller that can performvoltage comparison, and the controller can then preferably apply one ormore of the control signals to break the serial path into parallelpaths, each having a lower forward voltage arrangement than V1, allowingthe LEDs in the parallel paths to be lit even when the voltage is low.

For example, when a single control signal CSi is activated, Gi isconducting and current through LED arrays D1 to Di will be controlled byGi. Also, switch Si is conducting and current is directly supplied fromV1 to LED arrays Di+1 to Dn. In this case, two parallel connectedcurrent paths are formed, i.e., current path from D1 to Di which iscontrolled by Gi and current path from Di+1 to Dn which is controlled byGn. If a further control signal CSj is activated, where j>i, the circuitwill change into three parallel connected current paths of D1 to Di,Di+1 to Dj, and Dj+1 to Dn which are controlled by Gi, Gj and Gnrespectively.

When all control signals CS1 to CSn are activated, all LED arrays D1 toDn will be parallel connected to V1 through current sources G1 to Gnrespectively. The creation of the different parallel paths permits theLEDs to be lit even when the input voltage V1 is low. For example, toallow for the lighting of LED arrays even at low input voltage V1, theactivation of the control signals can be controlled such that for thelowest input voltage, the greatest number of parallel paths is formed,each path having a forward voltage that can be lit by the present inputvoltage. As the input voltage V1 increases, a smaller number of parallelpaths can be formed by application of control signals as describedabove, each path having more LED arrays, until, above a certain voltage,e.g., a voltage greater than or equal to the forward voltage of LEDarrays D1 to Dn, a single string of LED arrays D1 to Dn is formed, whichin the above example, would be when no control signals are activated.

The control signals can be generated by voltage comparators whichcompares the voltage of V1 to certain threshold voltage or currentsensors which sense the currents through the current sources. Moresophisticated control can be implemented with the use of amicrocontroller. FIG. 17 shows the functional block diagram of amicrocontroller that can be used with the circuit of FIG. 16. In FIG.17, V_(V1) and I_(V1) denote the voltage across V1 and the currentthrough V1 respectively. V_(Gi) and I_(Gi) denote the voltage across Giand the current through Gi respectively. The microcontroller preferablysamples and processes the various voltage/current signals and generatescontrol signals CS1 to CSn according to algorithms that are designed tooptimize efficiency, input power quality, LED arrays usage and lightoutput uniformity, etc.

For example, a simple example of such an algorithm is to keep thevoltage difference between V1 and the forward voltage of combined LEDarrays small in order to maximize efficiency. It is assumed the forwardvoltages of all LED arrays D1 to Dn are equal to same value Vd andmaximum of V1 is higher than the forward voltage sum of D1 to Dn, i.e.nVd. When V1<2Vd, all control signals are activated and D1 to Dn areparallel connected through G1 to Gn respectively. When 2Vd≦V1<3Vd, onlycontrol signals CSi are activated where i is even and i≦n. When3Vd≦V1<4Vd, only control signals CSi are activated where i is multipleof 3 and i≦n. When jVd≦V1<(j+1)Vd, only control signals CSi areactivated where i is multiple of j and i≦n. When nVd≦V1, all controlsignals are de-activated and D1 to Dn are connected in series throughcurrent source Gn. This is only one example and the invention is notlimited to this exemplary embodiment.

Also, the microcontroller can be programmed to have fault handlingability, e.g., the microcontroller can detect any faulted LED array andre-arrange the switching sequence to exclude the faulted LED array. Forexample, the microcontroller can be programmed so that if Di has a shortcircuit fault, control signal CSi−1 will be permanently de-activated sothat Di−1 and Di can be considered as a single array. If Di has an opencircuit fault, current will no longer flow through Di and control signalCSi will be permanently activated in order to have current supplied fromV1 to Di+1

FIG. 18 shows an example circuit of control signals generated by voltagecomparator and current sensor. In the circuit, current source G1 andswitch S1 are controlled by voltage comparator X1. Current source G2 canbe disabled by current sense signal from current source G3. A referencevoltage source, Vref, is coupled to the voltage comparator X1. It shouldbe noted that in this exemplary circuit D2 and D3 are directly connectedin series without any diode in between. It is because only two parallelcurrent branches (D1 and D2+D3) are needed in this example and thusthere is no need for connecting a switch and a blocking diode to theanode of D3.

For explanation purposes, it is assumed that the forward voltage of LEDarray D1 is larger than the forward voltage sum of D2 and D3, howeverthis is not required. The operation of the circuit shown in FIG. 18 isnext shown for the case in which the voltage of V1 is ramping up fromzero. While the voltage of V1 is less than the reference voltage Vref,comparator X1 outputs an active signal which enables both current sourceG1 and switch S1. When the voltage of voltage source V1 is just abovethe forward voltage of D2, current begins to flow through switch S1, LEDarray D2 and current source G2 as shown in FIG. 19A. Current source G2regulates the current through LED array D2 as voltage of V1 is furtherincreased.

LED array D3 begins to conduct through current source G3 when V1 reachesthe sum of the forward voltages of LED arrays D2 and D3, as shown inFIG. 19B. As the current through LED array D3 and current source G3 isincreasing to a threshold value, preferably lower than the regulatingvalue of current source G3, current source G2 is disabled, as shown inFIG. 19C. LED array D1 begins to conduct through current source G1 as V1gets higher to the forward voltage of D1, as shown in FIG. 19D.

It is preferable to set Vref to be slightly larger than the sum offorward voltages of LED arrays D1 and D2. FIG. 19E shows the currentpath when V1 is increased to Vref or above. In this case, switch S1 andcurrent source G1 are set to an open circuit condition by voltagecomparator X1. Current flows through LED array D1, diode Db1 (whichprevents back directional current), LED array D2 and current source G2.Further increasing V1 causes LED array D3 to conduct, as shown in FIG.19F. FIG. 19G shows the current path when the current through LED arrayD3 is increased to trigger current source G2 to be in the opencondition.

As would be understood by one of skill in the art, the switchingsequence shown in FIGS. 19A-19G would be reversed if the voltage of V1is declining. In particular, the situation in which the voltage of V1 ishigh enough to pass a regulated current through LED arrays D1 to D3 andcurrent source G3 as shown in FIG. 19G. As V1 is decreased the currentthrough current source G3 starts to decrease and to a point below thethreshold value, current source G2 is enabled and current begins to flowthrough current source G2, as shown in FIG. 19F. When V1 decreases to avalue below the sum of forward voltage sum of LED arrays D1 to D3,current through LED array D3 is stopped, as shown in FIG. 19E.

As V1 is further decreased to below Vref, switch S1 and current sourceG1 are enabled to conduct. Current through LED array D1 is regulated bycurrent source G1. Current through LED arrays D2 and D3 is regulated bycurrent source G3. Further decreasing V1 causes current through currentsource G1 to decrease to zero. When the current through current sourceG3 is decreased to a point below the threshold value, current source G2is enabled, as shown in FIG. 19B. When V1 is decreased to below the sumof forward voltages of LED arrays D2 and D3, current can only flowthrough LED array D2 and current source G2, as shown in FIG. 19A.

As can be seen from the above, the design of the circuit shown in FIG.18 provides for a period of driving of all of the LED arrays, inparallel, even during the period of time that the voltage of the voltagesupply is below Vref. This provides an improvement in the supply ofcurrent to the LED arrays and hence light output during low voltageoperation as compared with the design of the first embodiment.

FIG. 20 shows a practical exemplary detailed implementation of theproposed circuit shown in FIG. 18. In the figure, the AC 230V mainsvoltage is a rectified signal. The voltage waveform across node A and Bis shown in FIG. 21. Three LED arrays D1-D3 are used. The forwardvoltage of LED array D1 is 150V and forward voltage of both LED arraysD2 and D3 are 75V in the illustrative embodiment.

As can be seen in the figure, transistor M1, resistors R1 and R11, andZener diode ZD1 form a current source (generally corresponding tocurrent source G1 in FIGS. 18 and 19) which drives LED array D1.Resistors R4, R14 and transistor Q4 form a voltage comparatorcorresponding to X1 in FIGS. 18 and 19. Transistor M2, resistors R2, R12and Zener diode ZD2 form a current source corresponding to transistor G2in FIGS. 18 and 19. Transistor M3, resistors R3, R33, R13 and Zenerdiode ZD3 form a current source corresponding to G3 in FIGS. 18 and 19.

When the rectified mains voltage is low, transistors M1 and Q6 areconducting such that LED array D1 are parallel connected with LED arraysD2 and D3. In the exemplary embodiment, when rectified mains voltagelevel is above 225VDC, transistor Q4 turns off transistor M1, transistorQ5 and in turn transistor Q6 making a series connection of LED arraysD1, D2 and D3. FIG. 22 is a diagram that shows the LED arrays that areconducting during a half AC cycle.

As can be seen in the diagram of FIG. 22, and as was also illustrated inthe description above relating to FIGS. 19A-19G, during the period oftime that the voltage of the voltage supply is equal to or greater thanthe reference voltage, a forward voltage string of serially connectedLED arrays is formed, which increases as the voltage of the voltagesupply continues to increase above the reference voltage, and which isshortened as the voltage begins to decline. In the illustration, theforward voltage string initially includes D1 and D2. As the voltage ofthe voltage supply approaches its peak, the forward voltage stringincludes D1-D3, and then, as the voltage of the voltage supplydecreases, the length of the forward voltage string is reduced to D1 andD2.

As also shown in the diagram, during a portion of each period in whichthe voltage of the voltage supply is below the reference voltage,current will flow through all of the LED arrays D1-D3, but this willoccur in a parallel configuration, with one branch having LED array D1,and the other branch having LED arrays D2 and D3, as discussed abovewith reference to FIGS. 19A-19G.

FIG. 23 shows the current waveform of LED array D1. Waveforms of LEDarrays D2 and D3 are shown in FIGS. 24 and 25 respectively. FIG. 26shows the light output waveform of all the LED arrays. It should benoted the off time during zero crossing of the AC mains voltage isshorter than that in FIG. 13. The full light output time is also longerthat that in FIG. 13.

FIG. 27 shows the input current waveform from AC mains power source. Thepower factor for the exemplary circuit is about 0.85. Throughout most ofthe half line cycle, the current is continuous which makes the circuitsuitable to work with triac dimmer. Efficiency of the illustratedexemplary circuit is about 84%.

An optional valley-fill passive power factor correction circuit can beadded to improve the power factor and remove the off period at the zerocrossing mentioned above. FIG. 28 shows an illustrative embodiment of anexemplary valley-fill circuit, which in use could be used to connect tonodes A and B of FIG. 20. In operation, the rectified mains voltagecharges the valley-fill capacitors through the path C1, D5, R7 and C2.When the rectified mains voltage drops below half of its peak value, C1begins to discharge through D7 and C2 begins to discharge through D6.

FIG. 29 shows the rectified mains voltage waveform with the exemplaryvalley-fill circuit. It can be seen that the voltage does not go below150VDC because of presence of the energy storage capacitors C1 and C2 inthe valley-fill circuit.

FIG. 30 shows the current waveform of LED array D1 using the valley-fillcircuit. It is noted that because of the valley-fill circuit, thewaveform contains no off period. FIG. 31 shows the current waveform ofLED array D2, when the valley-fill circuit is used, which is similar towaveform of LED array D1. It should be noted that since the rectifiedvoltage is always, in the exemplary circuit, above 150V (the sum offorward voltage of LED arrays D2 and D3 in the example), the stage shownin FIG. 19A above will never occur when using the valley-fill circuit.FIG. 32 shows the current waveform of LED array D3 using the exemplaryvalley-fill circuit. FIG. 33 shows the light output waveform of all ofthe LED arrays. It should be noted by virtue of the valley-fill circuit,there is no off period in the light output.

FIG. 34 shows the input current waveform from AC mains power supplyusing the exemplary valley-fill circuit. The power factor is improved to0.9, while the efficiency of the circuit is kept at about 84%.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat a variety of alternate and/or equivalent implementations may besubstituted for the specific embodiments shown and described withoutdeparting from the scope of the present invention. This application isintended to cover any adaptations or variations of the specificembodiments discussed herein. Therefore, it is intended that thisinvention be limited only by the claims and the equivalents thereof.

What is claimed is:
 1. An LED array switching apparatus, comprising: aplurality of LED arrays arranged in a serial path, each LED array havinga forward voltage; a voltage supply coupled to the plurality of LEDarrays; a plurality of current sources selectively coupled to the LEDarrays, each of the current sources being switchable between a currentregulating state and an open state; and a controller that outputs atleast one control signal generated based on at least one of: (a) atleast one comparison between the voltage of the voltage supply and atleast one reference voltage, and (b) currents through one or more of thecurrent sources, the control signals controlling the turning on and offof at least one switch and a current source associated with the at leastone switch, the controller, the at least one switch and current sourcescooperating together such that: when the voltage of the voltage sourceis below the at least one reference voltage, and/or when a predeterminedlevel of current passes through the one or more current sources, atleast one switch is closed and one or more associated current sourcesare controlled so as to break the serial path into one or more parallelpaths each including less than all of the LED arrays.
 2. The LED arrayswitching apparatus according to claim 1, wherein for at least a portionof time during which the voltage of the voltage supply is below at leastone reference voltage, the one or more parallel paths comprise aplurality of parallel paths each including at least one of the LEDarrays, the plurality of parallel paths supplying current to all of theLED arrays.
 3. The LED array switching apparatus according to claim 1,further comprising: at least one diode arranged in the serial path ofthe LED arrays intermediate between a first group of LED arrays and asecond group of LED arrays; and a switchable parallel current path thatconnects the voltage supply to a point in the serial path between thediode and the second group of LED arrays, the at least one diodepreventing current from the parallel current path from flowing in thedirection of the first group of LED arrays.
 4. The LED array switchingapparatus according to claim 3, wherein the number of LED arrays in thefirst group of LED arrays is equal to the number of LED arrays in thesecond group of LED arrays.
 5. The LED array switching apparatusaccording to claim 3, wherein a plurality of parallel paths suppliescurrent to all of the LED arrays when the voltage of the voltage sourceis higher than the forward voltage of both of the first or second groupof LED arrays, but is less than the at least one reference voltage. 6.The LED array switching apparatus according to claim 1, wherein thevoltage source is a rectified AC voltage, the switching apparatusfurther comprising: valley-fill circuitry configured to preventoccurrence of any off period of light output at a zero crossing portionof the AC voltage.
 7. The LED array switching apparatus according toclaim 6, wherein the valley-fill circuitry includes at least one energystorage capacitor that discharges when the rectified AC voltage dropsbelow half its peak value to prevent any off period of the light output.8. The LED array switching apparatus according to claim 1, wherein atleast one of the plurality of LED arrays comprises a plurality of LEDs.9. The LED array switching apparatus according to claim 8, wherein theplurality of LEDs forming the at least one of the plurality of LEDarrays are arranged in parallel.
 10. The LED array switching apparatusaccording to claim 1, wherein the controller comprises one or morevoltage comparators.
 11. The LED array switching apparatus according toclaim 1, wherein the controller comprises a microcontroller.
 12. The LEDarray switching apparatus according to claim 11, wherein themicrocontroller is configured to detect a fault in an LED array andmodify a switching sequence to exclude the faulted LED array.
 13. TheLED array switching apparatus according to claim 1, wherein the at leastone reference voltage is a plurality of reference voltages, and the atleast one switch is a plurality of switches, and each of the pluralityof reference voltages corresponds with a respective one of the switches.14. An LED array switching apparatus, comprising: a plurality of LEDarrays arranged in a serial path, each LED array having a forwardvoltage; a voltage supply coupled to the plurality of LED arrays; avoltage comparator that compares the voltage of the voltage supply witha reference voltage and controls a switch to turn off when the voltageof the voltage supply is greater than or equal to the reference voltage;and a plurality of current sources selectively coupled to the LED arrayseach of the current sources is switchable between a current regulatingstate and an open state, the voltage comparator, the switch and currentsources cooperating together such that: (a) when the voltage of thevoltage source is below the reference voltage, the switch is closed andthe current sources are controlled so as to break the serial path intoone or more parallel paths each including less than all of the LEDarrays, and (b) when the voltage of the voltage supply is greater thanor equal to the reference voltage, as the voltage of the voltage supplyincreases, LED arrays are switched on and lit to form a higher forwardvoltage LED string, and as the voltage of the voltage supply decreases,LED arrays are switched off and removed from the LED string startingwith the most recently lit array.
 15. The LED array switching apparatusaccording to claim 14, wherein for at least a portion of time duringwhich the voltage of the voltage supply is below the reference voltage,the one or more parallel paths comprise a plurality of parallel pathseach including at least one of the LED arrays, the plurality of parallelpaths supplying current to all of the LED arrays.
 16. The LED arrayswitching apparatus according to claim 14, further comprising: a diodearranged in the series path of the LED arrays intermediate between afirst group of LED arrays and a second group of LED arrays; and aswitchable parallel current path that connects the voltage supply to apoint in the series path between the diode and the second group of LEDarrays, the diode preventing current from the parallel current path fromflowing in the direction of the first group of LED arrays.
 17. The LEDarray switching apparatus according to claim 16, wherein the number ofLED arrays in the first group of LED arrays is equal to the number ofLED arrays in the second group of LED arrays.
 18. The LED arrayswitching apparatus according to claim 16, wherein a plurality ofparallel paths supplies current to all of the LED arrays when thevoltage of the voltage source is higher than the forward voltage of bothof the first or second group of LED arrays, but is less than thereference voltage.
 19. The LED array switching apparatus according toclaim 14, wherein the voltage source is a rectified AC voltage, theswitching apparatus further comprising: valley-fill circuitry configuredto prevent occurrence of any off period of light output at a zerocrossing portion of the AC voltage.
 20. The LED array switchingapparatus according to claim 19, wherein the valley-fill circuitryincludes at least one energy storage capacitor that discharges when therectified AC voltage drops below half its peak value to prevent any offperiod of the light output.
 21. The LED array switching apparatusaccording to claim 16, wherein the sum of the forward voltages of LEDarrays in the first group of LED arrays is approximately equal to sum ofthe forward voltages of the LED arrays in the second group of LEDarrays.
 22. The LED array switching apparatus according to claim 14,wherein at least one of the plurality of LED arrays comprises aplurality of LEDs.
 23. The LED array switching apparatus according toclaim 22, wherein the plurality of LEDs forming the at least one of theplurality of LED arrays are arranged in parallel.